Method of crucible-free zone-melting of semiconductor material



Dec. 14, 1965 w. KELLER ETAL 3,223,493

METHOD OF CRUCIBLE-FREE ZONE-MELTING 0F SEMICONDUCTOR MATERIAL FiledJune 9. 1961 United States Patent 3,223,493 METHOD OF CRUClBLE-FREEZONE-MEETING 0F SEMICONDUCTOR MATERIAL Wolfgang Keller, Pretzfeld, andHerbert Kramer, Forehheim, Germany, assignors to Siemens-SchuckertwerkeAktiengesellschaft, Berlin-Siemensstadt, Germany, a corporation ofGermany Filed June 9, 1961, Ser. No. 115,974 Claims priority,application Germany, June 11, 1960, S 68,896; Nov. 2, 1960, S 71,088 3Claims. (Cl. 23-301) Our invention relates to a method of producing orprocessing hyperpure semiconductor material by a crucible-free orfloating zone-melting method, such hyperpure simiconductor materialbeing applicable in the production of rectifiers, transistors,photo-diodes, four-layer devices and other electronic semiconductorcomponents.

As a rule, the crucible-free zone-melting is performed by mounting a rodof the semiconductor material, preferably vertically, between twoholders which engage the respective ends of the rod. A ring-shapedheating device, in most cases an induction coil, surrounds thesemiconduction rod and, during zone-melting operation, travels axiallyalong and over the entire exposed length of the rod while melting anaxially narrow zone of the rod located within, and travelling with, theheater coil. In the travelling direction of the molten zone, newsemiconductor material is continuously melted at the leading side of thezone, whereas previously melted semiconductor material solidifies at thelagging side. Due to the crystalline structure of the material, theprogressing melting and recrystallizing has a purifying effect whicheliminates impurities from the rod material and causes it to migratetoward one end of the rod where it can subsequently be romoved bycutting a piece of the rod. The rod is thereafter sliced into Wafers foruse in respective semiconductor devices.

Another application of crucible-free zone-melting methods is the growingof monocrystals. For this purpose, a monocrystalline seed is fused toone end of a polycrystalline semiconductor rod, and a melting zone,starting from the fusion point, is passed through the entire length ofthe semiconductor rod toward the other end, this method being repeatedseveral times if desired.

The crucible-free zone-melting is also applicable for zone levelling. Incontrast to the purifying or monocrystal-growing methods, the moltenzone is repeatedly passed through the entire length of the rod in bothdirections so that the concentration of certain impurities, contained inthe semiconductor material or added thereto, is uniformly distributedover the entire rod length.

FIG. 1 shows apparatus for crucible-free zone-meltmg.

As a rule, the crucible-free zone-melting is performed within aprotective gas atmosphere or in an evacuated vessel. In the latter case,the above-described phenomena are accompanied by an additional purifyingeffect due to evaporation of foreign impurities from the rod into thevacuum.

While our invention is more specifically described with relation tocrucible-free zone-melting of silicon, other semiconductor material,such as germanium and llIV compounds e.g. gallium arsenide are alsoimproved by our invention, which fundamentally involves theabove-mentioned principles of the known methods, namely the use of anevacuated vessel in which a rod-shaped semiconductor body is mounted,preferably in vertical position, and in which the molten zone is passedseveral times along, and over the entire exposed length, of the rod, thepasses being either all in the ICC same direction or in oppositedirections depending upon the particular purpose.

According to the figure, apparatus for crucible-free zone-meltingcomprises a vessel composed of hell 31 of relatively great Width whichis vacuum-tightly seated upon a bottom plate 32. The bell 31 may consistof steel or other sheet metal and is preferably provided with anobservation window 31a of glass. The processing space within the vesselcan be evacuated through a nipple 31b to be connected to a vacuum pump.When desired water vapor containing gas may also be supplied through thenipple, or through a separate inlet (not shown) to the vessel. Mountedon the bottom plate 32 of the vessel is a frame structure 21 whichcarries a holder 22 vertically above another holder 23 that is mountedon the bottom plate 32 and may be fastened to a shaft 25 passing througha seal 33 in the bottom plate in order to permit imparting rotation tothe semiconductor material 24 during the zone-melting operation, as isdesirable for some purposes.

The semiconductor rod 24, for example of silicon, is held in and betweenthe two holders 22 and 23 and is surrounded by an inductive heating coil26. During operation of the device, with rod 24 inserted as shown andthe processing space evacuated, the coil 26 is energized by electriccurrent and causes a narrow horizontal zone of the rod 24 to melt, whilebeing slowly shifted upwardly or downwardly along the rod thus graduallypassing the melted zone through the entire body for the purpose ofpurifying it, levelling it and/or converting it to monocrystallinecondition.

Coolant, such as water, passes through the turns of the heater coil. Theentrance and exit of the flowing coolant are indicated in FIG. 1 byarrows 39.

According to our invention, we introduce into the processing vessel fora short interval of time, a water-containing gas, for example air fromthe environment or steam, prior to commencing the last pass of themelting zone.

We have discovered that by performing the cruciblefree zone-melting inthe manner just described, the resulting semiconductor material exhibitsgreatly improved properties, due to the fact that the lifetime of theminority carriers in the product is considerably increased. We have beenunable to fully explain this phenomenon. However, We consider itreasonable to assume that a slight amount of oxygen is built into theresulting crystal lattice and thereby eliminates lattice disturbances,particularly dislocations.

It is impossible to perform the crucible-free zonemelting in the air ofthe normal atmosphere because at the high processing temperatures(germanium melts at 930 C., silicon at 1430 C.) a disturbance in thecourse of the process and in the resulting crystal lattice takes placeby formation of oxides and other compounds. However, if the process iscarried out in high vacuum, using a high-vacuum vessel connected tocontinuously operating pumps that preserve the necessary vacuum, thenany impurities emerging from the semiconductor material are continuouslyexhausted from the processing space.

For example, when operating with high vacuum, for example of about 10"mm. Hg with a total of 10 passes of the molten zone along a silicon rod,the silicon product can be obtained with a specific resistance of 1000ohm cm. and an eifective charge-carrier lifetime T (tau of about 300 to500 seconds. These data relate to the use of starting materialconsisting of silicon already produced in hyperpure form, for example bypyrolytic precipitation of silicon from the gaseous phase onto a heatedsilicon core rod. All passes commence at the end of the silicon rod towhich the crystal tube was fused,

Patented Dec. 14, 1965 and all phases terminate at the opposite end.Whereafter the heating is reduced to a low temperature at which thesemiconductor material can solidify, but the material is still. inglowing condition, and the glowing zone is then passed in reversedirection back to the starting point of the next pass.

It now, according to the invention, prior to performing the last pass ofthe melting zone, the processing is interrupted and the vacuum vesselfilled for a short interval of time with water-containing gas, forexample atmospheric air, then the material resulting after completionofthe subsequent last melting-zone pass exhibits a lifetime T about fivetimesgreater than otherwise obtained, all other essential propertiesbeing not discernibly afiected.

Inpractice, the process can be carried out for example as follows. Afterthe ninth pass of the melting zone, the heating of the melting zone iscompletely discontinued, for example by de-energizingthe electricinduction coil. Then the vacuum vessel is permitted to cool for a shortperiod of time and rinsed with air. Thereafter the vacuum vessel isagain closed, sealed'and evacuated. Thereafter the tenth and last passof the melting zone is performed. Whenever the vacuum vessel isopened'in the course of the process, that is for the purpose ofsupplying air or upon completing the last pass, it is preferable toprovide for a sufficient cooling time of the semiconductor material toprevent the formation of oxide skins. In practice, we found it to besufiicient if after de-energizing the heating device, a waiting periodof approximatelyone minute is observed before opening the vessel. Thevessel may then be kept open for about 30 secondsin order to admit theingress of air or other humid atmosphere after the penultimatezone-melting pass.

When using steam instead of air, the process can be carriedout in asimilar manner. That is, after the melting zone has been passed throughthe rod the desired number of times, the melting zone is permitted tocool, steam is admitted into the vacuum vessel, whereafter the vessel isagain evacuated, and ultimately a melting zone is again produced and ispassed once through the entire length of the rod. The lattermodification is sometimes preferable and simpler in performance becausewater can readily be produced in the high degree of purity suitable forsemicondutor techniques.

The above-describedexamples can be modified in various respects. Thus,the number of zone passes is not critical and can be adapted to theother requirements or desiderata of the process; Critical only is thetemporary admission of water-containing gas prior'to the lastzonemelting pass. It is of course possible to repeat the justmentionedprocessing step by opening the vacuum vessel and admitting air or steam.For example, the vacuum vessel could be supplied with water-containinggas after each individual zone-melting pass. However, we have found thatno further improvement is obtained in this manner. Hence, we found it tobe sufilcient and most economical to admit water-containing air into thevacuum vessel only prior to performing the last zone-melting pass.

It should be noted that if water-containing gas were introduced duringthe zone-melting pass, there would be continuous electric discharges atthe heater coil within the vacuum vessel, which is undesirable since itwould' introduce uncontrolled amounts of impurities from the coil andmake the rod unsuitable for semiconductor purposes.

We claim:

1. In the method of crucible-free zone-melting of silicon semiconductormaterial wherein arod-shaped semiconductor bodyis mounted within avacuum vessel, said vessel being evacuated, and a heater within said'vessel serves to cause a molten zone to pass repeatedly through andalong the length of said body being within said vessel, the improvementwhich comprises the steps of introducing humid air into said vesselafter the penultimate zone-melting pass, re-evacuating the vessel andperforming the last zone-melting'pass.

2. In the method of crucible-free zone-melting of silicon semiconductormaterial wherein a rod-shaped silicon body is mounted within a vacuumvessel, said vessel being evacuated, and a heater within said vesselserves to cause a molten zone to pass repeatedly. through and along thelength of said body being within said vessel, the improvement whichcomprises the steps of introducing a water-containing gas into saidvessel after the penultimate zone-melting pass, re-evacuating the vesseland performing the last zone-melting pass.

3.. In the method of crucible-free zone-melting of silicon semiconductormaterial wherein a rod-shaped silicon body is mounted within a vacuumvessel, said vessel beingevacuated, and a heater within said vesselserves to cause a molten zone to pass repeatedly through and along thelength-of said body being within said vessel, the improvement whichcomprises the steps of introducing steam into said vessel after thepenultimate zone-melting pass,

re-evacuating the vessel and performing the last zonemelting pass.

References Cited by the Examiner UNITED STATES PATENTS 2,901,325 8/1959Theurer 23-301 X OTHER REFERENCES Burris et al.: Contribution to theMathematics of Zone Melting, Argonne National Laboratory, July 7, 1954.

Wertheim et al.: Electron-Bombardment Damage in Oxygen-Free Silicon,Journal of Applied Physics, vol. 30, No. 8, August 1959; pp. 1232-1234.

NORMAN YUDKOFF, Primaly Examiner.

RAY K. WINDHAM, Ekaminer.

1. IN THE METHOD OF CRUCIBLE-FREE ZONE-MELTING OF SILICON SEMICONDUCTORMATERIAL WHEREIN A ROD-SHAPED SEMICONDUCTOR BODY IS MOUNTED WITHIN AVACUUM VESSEL, SAID VESEL BEING EVACUATED, AND A HEATER WITHIN SAIDVESSEL SERVES TO CAUSE A MOLTEN ZONE TO PASS REPEATEDLY THROUGH ANDALONG THE LENGTH OF SAID BODY BEING WITHIN SAID VESSEL, THE IMPROVEMENTWHICH COMPRISES THE STEPS OF INTRODUCING HUMID AIR INTO SAID VESSELAFTER THE PENULTIMATE ZONE-MELTING PASS, RE-EVACUATING THE VESSEL ANDPERFORMING THE LAST ZONE-MELTING PASS.